Many mechanical systems have components that come in and out of contact with each other during operation. The contact points between two such components are sources of “wear particles” that are generated in three ways: abrasion, adhesion, and fatigue. These types of physical interaction occur when the components come into contact with each other and then when the components separate from each other. Wear particles are small particles that break away from the body of a component and are then free to float around a system. Wear particles can be very small in size, but they can present problems in terms of contaminating clean system operating environments.
Wear particles are generated through abrasion when any amount of rubbing between two components causes one component to scratch and/or cut off particles from the other component. Typically, the component that is formed of the harder material will cause particles to be rubbed off of the softer component. Even very small amounts of rubbing can cause wear particles to form through abrasion. Adhesion can cause wear particles when the material of a first component adheres to the material of a second component and is pulled out from the main body of the first component upon separation of the components. With adhesion, the softer material tends to adhere to the harder material and then gets pulled away. However, portions of a harder material can also get pulled out of its body by a softer material. Fatigue can cause wear particle generation because the cyclical contact between two components can cause flexing or deformation of a material to the point where particles start to break away.
FIG. 1 illustrates a diagrammatic view of a mechanical system 100 wherein two components, component A 102 and component B 104 come in and out of contact with each other such that wear particles 106 are generated. Notice that the wear particles 106 float around and contaminate a process device 108 within mechanical system 100. Wear particles 106 can be referred to as airborne or gas-borne particles as they can be small enough to float through a system. Again, wear particles 106 are broken away from components A and B, 102 and 104, because the components come in and out of contact with each other. Wear particles 106 can come from component A 102 and/or component B 104.
Mechanical system 100 can be in any type of application. For example, one application is a semiconductor manufacturing system wherein process device 108 is one of the subsystems that can be adversely affected by wear particles 106. One such process device 108 is a reticle handling system used in photolithographic processes. In photolithography, reticles are the templates, which are used to optically copy an image of a circuit pattern onto a film, known as a photoresist. Reticles can be formed of a quartz plate that has a chrome-patterned region formed on one of the reticle surfaces. A defect on the reticle will appear as a defect on the transferred image in the photoresist. Wear particles generated from mechanical components that make contact with each other are one source of such defects on a reticle. For example, an end-effector used to manipulate a reticle will come into contact with a reticle. An end-effector is, for example, a robotic arm. Then at a later time, the end-effector will disconnect from the reticle such that particles are formed. This type of physical contact is just one of the many sources of wear particle generation in a semiconductor manufacturing system.
In conventional photolithography systems, reticles are protected from particle contamination by a pellicle. A pellicle is a thin, optically clear membrane, which covers the pattern on the reticle. The pellicle can be easily cleaned without damaging the reticle pattern. The pellicle supported over the patterned area of the reticle by a surrounding wall. The wall and pellicle enclose a small interior volume over the patterned region of the reticle. The opposite surface of the reticle plate, which is not patterned is not protected because, without any patterning, the surface is easy to clean. Also, particles that fall on the opposite surface of the reticle from the pellicle are outside the optical system focal range and do not show up as defects on the photoresist. Typically, there is a filtered opening in the surrounding wall that allows for pressure equalization within the interior volume created by the pellicle and the wall. The filter also allows for gas flow in and out of the interior volume without the introduction of particles that would be large enough to be considered as defects.
Although pellicles have proven to be effective devices for protecting reticles from contamination, future-generations of semiconductor manufacturing systems will employ extreme ultraviolet (EUV) lithography, which uses a smaller wavelength light capable of higher resolution image transfer. Unfortunately, there exists no practical pellicle for protecting EUV reticles. One reason for this is that the extremely small wavelength of light used in EUV systems are easily absorbed by many mediums, including the pellicle. Use of the pellicle would be impracticable because extremely high intensity light sources would be needed to achieve a practical manufacturing throughput. An exemplary wavelength of light used by EUV systems is approximately 13 nm. Therefore, the reticle handling system for EUV systems will require new techniques for protecting the reticle from particles.
Much effort has been put forth to find techniques for reducing wear particle generation in all types of semiconductor manufacturing systems. One approach is to carefully select the material that forms components that come in contact with each other. For instance, it has been found that polyimide material in contact with a reticle (either bare ULE™ or Chrome (Cr) coating) generates a low number of airborne or gas-borne particles. In contrast to polyetheretherketone (PEEK, which is typically used in end-effectors) generates a relatively large number of airborne or gas-borne particles.
Efforts have also been concentrated on preventing such particles from reaching critical process components, such as patterned regions of a reticle. Several techniques have been suggested, including photophoresis, electrophoresis, thermophoresis, magnetophoresis, electrically grounding a reticle to its to surroundings, using removable covers, and removable pellicles. Unfortunately, all of the techniques for protecting reticles from airborne or gas-borne particles have shortcomings.
Photophoresis uses a relatively weak force and requires a powerful optical source to drive particles away from the reticle. Electrophoresis, magnetophoresis, and electrical grounding are not effective on electrically neutral particles. Removable covers and pellicles do not completely block the reticle and generate particles during contact events with the reticle or end-effector. Thermophoresis is not effective in high vacuum environments and is not effective on particles where Brownian forces dominate particle motion.
Some have suggested eliminating particle generation altogether by either using electrostatic and/or electromagnetic techniques of levitating and moving the reticle without physical contact. These non-contact handling techniques also have shortcomings. Electrostatic levitation systems have limited reticle transfer rates because of weak levitation forces, the accompanying electrostatic fields may drive charged particles to the reticle and exacerbate contamination, and implementation of such systems drastically increases the costs of reticle handling systems. Electromagnetic levitation systems generate large amounts of heat, makes reticle handling systems significantly more expensive, and require adding magnetic material to reticles, which adds additional process steps to reticle manufacturing processes and increases reticle costs.
In view of the foregoing, there are continuing efforts to provide improved systems that have components that come in and out of contact with each other where the components do not generate wear particles. Efforts also continue in providing improved techniques for preventing these wear particles from migrating to surrounding process components that may be adversely affected by the particles.